U.S. Department of Energy Secretary Steven Chu announced Friday that he is resigning from his role, but a successor has not been named.

Chu, a scientist and Nobel Prize winner who assumed office in January 2009 as the 12th U.S. Secretary of Energy, had planned to leave for a few months now. When President Barack Obama took office after the November 2012 election, Chu told Obama that he wanted to return to his academic life in California, where he served as director of the Lawrence Berkeley National Laboratory.

Chu wrote a letter of resignation, which was published on Friday. He listed the department's many accomplishments while he was in office, such as the doubling of clean, renewable energy from wind and solar; helping one million low-income homeowners weatherize their homes; building, opening and retooling more than a dozen auto manufacturing plants for hybrids and EVs; launching the first national scale rooftop solar project, which will include commercial buildings in up to 28 states, and giving grants/loans to over 1,300 companies through the Recovery Act.

However, Chu's time as secretary hasn't been all roses and rainbows. The Department of Energy had been given $36 billion in stimulus money to spend, and Chu was criticized for giving some of the money to clean energy companies as loan guarantees and grants when they eventually filed for bankruptcy.

One of the most notable failures was Solyndra, a Silicon Valley-based solar panel company that received $535 million from DOE in 2009. The move was set to stimulate economic growth through environmentally friendly jobs, but in September 2011, Solyndra declared bankruptcy -- laying off 1,100 workers.

The biggest issue was that the money was given despite previous warnings of Solyndra's viability. Only days before the final approval, an email predicted that the project would run out of money in 2011. Another email from government staffers questioned the model the government was using, but said "given the time pressure we are under to sign off on Solyndra, we don't have time to change the model."

Reports state that the White House pushed the loan ahead despite warnings in order to meet political deadlines.

There were other green energy loan failures too, such as a $43 million loan guarantee to Beacon Power in 2010 (the company filed for bankruptcy in November 2011) and a $118.5 million grant to Ener1 Inc.'s subsidiary EnerDel in 2009 (Ener1 Inc. filed for bankruptcy in January 2012).

Nevertheless, Chu has made great strides in efficient energy in the U.S. He said only 1 percent of the 1,300 companies filed for bankruptcy. He's also pushing for the continued support of green energy into the future.

"The test for America's policy makers will be whether they are willing to accept a few failures in exchange for many successes," wrote Chu.

Chu plans to keep his role through the end of February, but will then return to California. A successor has not been named yet, but a few candidates are former North Dakota Sen. Byron Dorgan, former Michigan Gov. Jennifer Granholm and former San Francisco hedge fund manager Tom Steyer.